You can
install PM onto two machines, just as long as you don't
use it on both machines at the same time. This at least is my understanding of what Kirk has been replying quite a few times before to similar questions.
Edit: Ah, Kirk beat me to my reply
To my knowledge you do not need any special patches for PM to run on Leopard (can't yet test as I don't yet own a Mac), just make sure you use the latest version of PM (4.5.3).